A 4-year-old spayed-female Domestic Shorthair cat, obtained from a shelter as a kitten, was examined in May 2007 because of an intermittent lameness. Rectal temperature was 1021F. The cat weighed 3.3 kg. There was a focal, nonpainful, 2.5-cm-diameter firm swelling at the medial aspect of the left metatarsal region. Radiographs of the left rear leg identified lysis of the distal aspect of the 1st metatarsal bone and adjacent soft tissue swelling. Thoracic radiographs were unremarkable. Neoplasia was suspected and digital amputation was elected. Preanesthetic blood tests identified thrombocytopenia (25,000/mL; reference range, 200,000-500,000) with an adequate estimated platelet count because of clumping, mild lymphopenia (1,071 lymphocytes/mL; reference range, 1,200-8,000), mild hyperglobulinemia (5.4 g/dL; reference range, 2.3-5.3), and negative FIV and FeLV ELISA test results. The neutrophil count was 4,725/mL (reference range, 2,500-8,500) with no bands or neutrophil toxicity. Urinalysis was unremarkable.At surgery, the left medial metatarsophalangeal joint was disarticulated and the digit was removed. Histopathologic evaluation identified a mixed inflammatory response, characterized by a large number of well-differentiated plasma cells, macrophages with erythrophagocytosis and focal aggregates of neutrophils. Pyogranulomatous osteomyelitis was diagnosed. Plasma cell numbers were sufficiently high that plasma cell neoplasia was considered a plausible differential diagnosis. Fungal, acid-fast, and silver stains did not identify infectious agents. Culture of the amputated tissue was not performed. Two weeks postoperatively, an ulcer formed at the surgical site and was accompanied by moderate muscle atrophy. The distal limb became necrotic from the surgical site to the mid-tibia, which was accompanied by severe atrophy of the left rear leg to the level of the stifle and a loss of distal deep pain, which required mid femoral amputation. At no time had the cat traumatized the original incision. Despite necrosis, there was minimal to no purulent discharge and the cat never appeared overtly uncomfortable, despite rapid deterioration in the limb.In November 2007, the cat was examined because of lethargy. The cat was thrombocytopenic (97,000/mL; reference range, 175,000-600,000) and neutropenic (1,064/mL; reference range, 2,500-8,500). Urinalysis identified proteinuria and neutrophils suggestive of a urinary tract infection and the cat was treated with amoxicillin-clavulanate 62.5 mg PO q12h for 7 days. Urine culture was not performed.In February 2008, diffuse distal swelling and lameness developed in the left front leg. Radiographs identified mild soft tissue swelling and mild degenerative joint disease. The cat was treated with meloxicam 0.45 mg PO q24h for 3 days, then q72h to alleviate pain and swelling. Despite this treatment, the cat remained intermittently lame. Swelling and lameness of the left front leg again were documented by physical examination in May 2008, at which time the swelling had become firmer and ...